1. Field of the Invention
This invention relates to light emitting diode (LED) chips and in particular LED chips having increased efficiency and light extraction.
2. Description of the Related Art
Light emitting diodes (LED or LEDs) are solid state devices that convert electric energy to light and generally comprise an active region of semiconductor material sandwiched between two oppositely doped layers of semiconductor material. When a bias is applied across the doped layers, holes and electrons are injected into the active region where they recombine to generate light. Light is emitted from the active layer and from all surfaces of the LED.
In order to use an LED chip in a circuit or other like arrangements, it is known to enclose an LED chip in a package to provide environmental and/or mechanical protection, color selection, light focusing and the like. An LED package can also include electrical leads, contacts or traces for electrically connecting the LED package to an external circuit. FIG. 1 shows a conventional LED package that generally comprises a single LED chip 12 mounted on a reflective cup 13 by means of a solder bond or conductive epoxy. One or more wire bonds 11 connect the ohmic contacts of the LED chip 12 to leads 15A and/or 15B, which may be attached to or integral with the reflective cup 13. The reflective cup 13 can be filled with an encapsulant material 16 which can contain a wavelength conversion material such as a phosphor. Light emitted by the LED at a first wavelength can be absorbed by the phosphor, which can responsively emit light at a second wavelength. The entire assembly is then encapsulated in a clear protective resin 14, which may be molded in the shape of a lens over the LED chip 12.
FIG. 2 shows another conventional LED package 20 that may be more suited for high power operations that can generate more heat. In the LED package 20, one or more LED chips 22 are mounted onto a carrier such as a printed circuit board (PCB) carrier, any other board, substrate or submount 23. A reflector 24 can be included on the submount 23 that surrounds the LED chip(s) 22 and reflects light emitted by the LED chips 22 away from the package 20. Different reflectors can be used such as metal reflectors, omni-directional reflectors (ODRs), and distributed Bragg reflectors (DBRs). The reflector 24 can also provide mechanical protection to the LED chips 22. One or more wire bond connections 11 are made between ohmic contacts on the LED chips 22 and electrical traces 25A, 25B on the submount 23. The mounted LED chips 22 are then covered with an encapsulant 26, which may provide environmental and mechanical protection to the chips while also acting as a lens. The metal reflector 24 is typically attached to the carrier by means of a solder or epoxy bond. In these embodiments the LED chips are generally mounted such that the top and bottom (on the carrier) surfaces have a larger surface area than the side surfaces. Therefore any light emitted towards the bottom of the LED chip must be reflected up and through the LED chip to exit the chip, resulting in extra reflections of the emitted light. Each extra bounce of light on its way to emission from the LED chip results in about 2% loss of light, when a highly reflective mirror is in use. On average, when LED chips are mounted in this configuration, light experiences 9 bounces before it can exit the LED chip.
LEDs can be fabricated to emit light in various colors. However, conventional LEDs cannot generate white light from their active layers. Light from a blue emitting LED has been converted to white light by surrounding the LED with a yellow phosphor, polymer or dye, with a typical phosphor being cerium-doped yttrium aluminum garnet (Ce:YAG). The surrounding phosphor material “down-converts” the energy of some of the LED's blue light which increases the wavelength of the light, changing its color to yellow. Some of the blue light passes through the phosphor without being converted while a portion of the light is down-converted to yellow. The LED emits both blue and yellow light, which combine to provide a white light. In another approach light from a violet or ultraviolet emitting LED has been converted to white light by surrounding the LED with multicolor phosphors or dyes.
In recent years, there have been dramatic improvements in light emitting diode technology such that LEDs of increased brightness and color fidelity have been introduced. Due to these improved LEDs, lighting modules have become available to further increase luminous flux output. Both single and multi-chip modules have become available, with a single-chip module generally comprising a single package with a single LED. Multi-chip lighting modules typically comprise a single package with a plurality of LEDs. These lighting modules, particularly the multi-chip modules, generally allow for high output of light emission.
However, the emitted light from the device chip(s) may be largely non-directional and non-uniform, which can negatively impact the emission and optical efficiency of a lighting module. Furthermore, traditionally these LEDs are mounted such that the active region is parallel to the mounting surface. Therefore, a mirror must be placed under the active region, or between the active region and the mounting surface to reflect the light emitted towards the mounting surface up and out of the light emitter. Due to emission of the light towards the mirror and total internal reflection, emitted light reflects several times before exiting the light emitter. Generally there is a 2-3% light loss per bounce or reflection. On an average, it takes about 8-9 reflections for photons to exit the LED. This can result in approximately total of 16-27% light loss due to multiple reflections.
Often, a light diffusion lens, light scattering particles, and/or phosphor particles are disposed or deposited over the chip(s) to assist in achieving more uniform light emission. A fraction of brightness can be lost when utilizing such means, largely due to back-emission from the emitter, or scattering and back-reflection of light from a light diffusion lens, light scattering particles, and phosphor particles.
To redirect the back-emitted, scattered and/or back-reflected light, reflective materials have been disposed or deposited on the substrate of various light emitting devices. The reflective materials may be deposited on only portions of the substrate, or may be disposed or deposited as a reflective layer on the substrate. In other attempts to redirect scattered and/or back-reflected light, light-reflective, white printed circuit board (PCB) and/or substrate technology has been developed. The materials used for this existing technology may be epoxy or silicone-based. Epoxy or Silicones may yellow during prolonged use and/or common fabrication steps known in the art, such as reflow soldering. Epoxy materials may also degrade in the presence of blue light.